The present invention relates generally to switching devices. More specifically, the present invention relates to improved packaging and circuit integration for electromagnetic devices, such as reed switches and electromagnetic devices, such as reed relays, for switching high frequency signals. These relays are intended for applications in industries such as Automated Testing Equipment (ATE), where test signals having frequency ranges from DC to 12 GHz must be switched with minimum power loss and minimum pulse distortion.
Electromagnetic relays have been known in the electronics industry for many years. Such electromagnetic relays include the reed relay which incorporates a reed switch. A reed switch is a magnetically activated device that typically includes two flat contact tongues which are merged in a hermetically sealed glass tube filled with a protective inert gas or vacuum. The switch is operated by an externally generated magnetic field, either from a coil or a permanent magnet. When the external magnetic field is enabled, the overlapping contact tongue ends attract each other and ultimately come into contact to close the switch. When the magnetic field is removed, the contact tongues demagnetize and spring back to return to their rest positions, thus opening the switch.
Reed switches, actuated by a magnetic coil, are typically housed within a bobbin or spool-like member. A coil of wire is wrapped about the outside of the bobbin and connected to a source of electric current. The current flowing through the coil creates the desired magnetic field to actuate the reed switch within the bobbin housing. Some applications of reed devices require the switch to carry signals with frequencies in excess of 500 MHz. For these applications, a ground shield conductor, commonly made of copper or brass is disposed about the body of the reed switch. The ground shield conductor is commonly in a cylindrical configuration. The shield conductor resides between the reed switch and the bobbin housing to form a co-axial high frequency transmission system. This co-axial system includes the outer shield conductor and the switch lead signal conductor co-axially through the center of the reed switch. The ground shield conductor is employed to contain the signal through the switch conductor in order to maintain the desired impedance of the signal path.
Currently available reed devices are then incorporated into a given circuit environment by users. For application at higher frequencies, a reed switch device must be ideally configured to match as closely as possible the desired impedance requirements of the circuit in which it is installed.
Within a circuit environment, a co-axial arrangement is preferred throughout the entire environment to maintain circuit integrity and the desired matched impedance. As stated above, the body of a reed switch includes the necessary co-axial environment. In addition, the signal trace on the user""s circuit board commonly includes a xe2x80x9cwave guidexe2x80x9d where two ground leads reside on opposing sides of the signal lead and in the same plane or a xe2x80x9cstrip linexe2x80x9d where a ground plane resides below the plane of the signal conductor. These techniques properly employed provide a two-dimensional, controlled impedance environment which is acceptable for maintaining the desired impedance for proper circuit function.
However, the reed switch device must be physically packaged and electrically interconnected to a circuit board carrying a given circuit configuration. It is common to terminate the shield and signal terminals to a lead frame architecture and enclose the entire assembly in a dielectric material like plastic for manufacturing and packaging ease. The external portion of the leads may be formed in a gull-wing or xe2x80x9cJxe2x80x9d shape for surface mount capability. The signal leads or terminals exit out of the reed switch body and into the air in order to make the electrical interconnection to the circuit board. This transition of the signal leads from plastic dielectric to air creates an undesirable discontinuity of the protective co-axial environment found within the body of the switch itself. Such discontinuity creates inaccuracy and uncertainty in the impedance of the reed switch device. As a result, circuit designers must compensate for this problem by specifically designing their circuits to accommodate and anticipate the inherent problems associated with the discontinuity of the protective co-axial environment and the degradation of the rated impedance of the reed switch device.
For example, the circuit may be tuned to compensate for the discontinuity by adding parasitic inductance and capacitance. This method of discontinuity compensation is not preferred because it complicates and slows the design process and can degrade the integrity of the circuit. There is a demand to reduce the need to tune the circuit as described above. The prior art uses a structure of carefully designed vias, which are expensive and difficult to manufacture, to control the impedance from the relay to the board transition.
There have been many attempts in the prior art to solve the aforementioned problems associated with the packaging and the incorporation of reed switch devices into a circuit. For example, prior art reed switch devices typically include a printed circuit board substrate onto which the reed switch itself is installed. Circuit board traces are deposited on the surface of the printed circuit board to provide a wave guide to extend the co-axial environment of the relay from the reed switch itself down to the main circuit board into which the device package is installed. However, there are problems associated with the use of a printed circuit board as a substrate within an overmolded device package as well as manufacturing limitations.
Since it is commonly desired that the reed switch package be as small as possible, the use of a very thin printed circuit board is required. While a thin printed circuit board substrate has good RF transmission characteristics, it is less than ideal mechanically. The epoxy/fiberglass material of a typical printed circuit board is thin and fragile, and is subject to distortion or cracking under the heat and pressure stresses of the encapsulation process. Distortion of the leads can lead to misalignment of the solder balls when they are fastened to the product after molding. If the misalignment is severe, one or more relay balls can miss the solder pads on the user""s circuit board when the relay is fastened, causing electrical discontinuities that require expensive rework.
The substrate solder pads are also fragile and are, therefore, easily damaged when the relay solder balls are reflow soldered to the substrate. A further disadvantage of solder pads is that they are flat; because of this, the solder balls can wander on them during attachment, causing further misalignment. After the relay is molded, solder balls are fixed to pads provided on the exposed external portions of the substrate traces. The solder balls melt when the relay is applied to the user""s circuit board, providing the electrical connections to the reed switch, coaxial shield and coil. Since the circuit board substrate has a fibrous edge profile that is exposed at the exterior of the relay, it also provides a potential path for ingress of moisture during circuit board cleaning processes. Water ingress is highly undesirable, since it can lower the relay""s insulation resistance. Also, the printed circuit board is relatively expensive compared to the total component cost for the entire product. Therefore, it is desired for this part to be removed from the construction.
In the prior art, there have been attempts to eliminate the use of printed circuit board substrates in electronic device packages. Many molded electronic packages use an internal metal leadframe skeleton to support internal components and transmit electrical signals in and out of the package. The leadframe supports the internal components during assembly, and is cut away after the product is molded leaving legs or pins that are used for external connections.
A metal leadframe could provide such features to obviate the need for a printed circuit board provided that it does not degrade the quality of the signals being transmitted through the relay. However, leadframes are generally not optimized for very high frequency signal transmission. At frequencies of several GHz and beyond, signals must be carried on special structures such as tuned striplines or waveguides to minimize losses. Known leadframe structures are not capable for accommodating such high frequency signals. In particular, known leadframe structures are not capable of meeting industry requirements for relays used for testing high speed memory and other semiconductors which is a loss of no more than half power (xe2x88x923 dB) for signals up to 5 GHz (5xc3x97109 Hz) which fall into the radio frequency (RF) band. The deficiencies in known leadframe capability will continue to be particularly inadequate in the future as the above requirement is likely to rise to 20 Ghz and beyond over the next few years.
In view of the foregoing, there is a demand for a reed switch device that includes a controlled impedance environment through the entire body of the package to the interconnection to a circuit. There is a particular demand for a reed switch device to be compact and of a low profile for installation into small spaces and for circuit board stacking. There is further a demand for reed switch devices that are of a surface mount configuration to optimize the high frequency of the performance of the system. Further, there is a demand for a reed switch device that can reduce the need to tune a circuit to compensate for an uncontrolled impedance environment. There is a further need for a reed relay package that is low in cost yet still robust and rugged in construction with the ability to transmit high frequency signals through a closed relay with minimum power loss.
The present invention preserves the advantages of prior art electromagnetic switch devices, such as reed relays. In addition, it provides new advantages not found in currently available switching devices and overcomes many disadvantages of such currently available devices.
The invention is generally directed to the novel and unique reed switch device with particular application in effectively interconnecting a reed switch device to a circuit on a circuit board in a low profile configuration. The reed switch package of the present invention enables the efficient and effective interconnection to a circuit board while being in an inexpensive construction.
The electromechanical device of the present invention mounts forms a low profile, board surface mountable reed relay. The reed device package includes a reed switch with two signal terminals emanating from opposing sides thereof. A leadframe is employed with signal conductors and ground conductors. The signal conductors are respectively attached to each of the signal terminals. A ground shield surrounds the body of the reed switch. The ground conductors are connected to the ground shield on a first side of the reed switch with the signal conductor on one side of the reed switch being positioned between the two ground conductors. Another pair of ground conductors are connected to the ground shield on the other side of the switch and are similarly positioned with the other signal conductor positioned therebetween. The reed switch device is overmolded with encapsulation material with the exception of the free ends of the signal and ground conductors which receive solder balls thereon for surface mount installation to a circuit board. After encapsulation, excess portions of the leadframe are trimmed away.
It is therefore an object of the present invention to provide a compact, low profile reed switch package.
It is an object of the present invention to provide a reed switch device with a controlled impedance environment throughout the entire package.
It is a further object of the present invention to provide a reed switch package with an improved substrate that is stronger and dimensionally more accurate than the existing printed circuit board substrates.
A further object of the present invention is provide a reed switch package that has a substrate that minimizes breakage and distortion during manufacturing.
Another object of the present invention is to provide a reed switch package that is capable of efficiently conducting high frequency signals.
It is a further object of the present invention to provide a reed switch package that is inexpensive to manufacture and more reliable to assemble.
It is yet a further object of the present invention to provide a reed switch package that has solder ball placement that meet coplanarity installation requirements.
Another object of the present invention is to provide a reed switch package that can be easily surface mounted to a main circuit board.
It is yet another object of the invention to provide a reed switch package with a metal substrate that is optimized for high frequency signal transmission.